From the viewpoint of the preservation of the global environment, global warming has been recently regarded as a problem, and it has been desired to reduce carbon dioxide CO2 emissions by such methods as improving the fuel efficiency of automobiles. From such a viewpoint of curbing global warming, hydraulic power generators have been recently reconsidered as a clean energy source. A generator such as the hydraulic power generator includes a rotor and a stator, in which the rotor includes a pole core serving as an iron core and a rim that supports it. In order to gain generating capacity, the rotor is required to be rotated at a high speed. For this purpose, the rim is required to hold high strength in order to resist a centrifugal force caused by the high-speed rotation, and hot-rolled steel sheets having a yield strength of about 550 MPa have been mainly used for the rim. However, it has been recently demanded to use high-strength hot-rolled steel sheets having a yield strength of about 700 MPa or more. The steel sheets for the rim are required to hold excellent magnetic properties at the same time.
In response to such a demand, Patent Literature 1, for example, discloses a hot-rolled steel sheet containing, in terms of percent by weight, C: 0.02% or more and 0.10% or less, Si: 2.0% or less, Mn: 0.5% or more and 2.0% or less, P: 0.08% or less, S: 0.006% or less, N: 0.005% or less, and Al: 0.01% or more and 0.1% or less, contains Ti in an amount of Ti: 0.06% or more and 0.3% or less and 0.50<(Ti-3.43N-1.5S)/4C, and having a microstructure that has an areal ratio of low-temperature transformed products and pearlite of 15% or less, and in which TiC is dispersed in polygonal ferrite. With the technique disclosed in Patent Literature 1, one or more of Nb, Mo, V, Zr, Cr, Ni, Ca, or other elements may be contained in the hot-rolled steel sheet. Although not considering magnetic properties, the technique disclosed in Patent Literature 1 can achieve a hot-rolled steel sheet having remarkably improved stretch flange formability at high strength with a tensile strength TS of 70 kgf/mm2 (690 MPa). However, the technique disclosed in Patent Literature 1 requires a large content of Ti in order to ensure the desired high strength. This makes coarse Ti carbide exceeding 30 nm, which does not contribute to higher strength, likely to be produced. The amount of solute Ti increases. Bainitic ferrite having high dislocation density is likely to be produced, and magnetic properties can degrade accordingly.
Patent Literature 2 discloses a method for manufacturing a high-tensile hot-rolled steel sheet having high magnetic flux density. The technique disclosed in Patent Literature 2 is a method for manufacturing a high-tensile hot-rolled steel sheet including heating a steel slab containing, in terms of percent by weight, C: 0.05% or more and 0.15% or less, Si: 0.50% or less, Mn: 0.70% or more and 2.00% or less, P: 0.020% or less, S: 0.010% or less, sol. Al: 0.010% or more and 0.10% or less, N: 0.0050% or less, Ti: 0.10% or more and 0.30% or less, and B: 0.0015% or more and 0.005% or less to a temperature of 1200° C. or more, performing hot rolling with a hot-rolling finishing temperature within the range of the Ar3 transformation point or more and 950° C. or less, cooling it with a cooling rate within the range of 30° C./s or more and less than 70° C./s, and winding it at 500° C. or less. The technique disclosed in Patent Literature 2 can achieve a high-tensile strength hot-rolled steel sheet having high magnetic flux density with a magnetic flux density B100 of 1.77 T or more with an yield strength YS of 80 kg/mm2 (785 MPa) or more and a tensile strength TS of 100 kg/mm2 (980 MPa) or more. However, the technique disclosed in Patent Literature 2 essentially contains B for the purpose of improving hardenability and performs quenching after hot rolling. This makes a bainite phase likely to be produced, and magnetic properties degrade, leading to insufficient magnetic properties as an iron core of a rotary machine.
Patent Literature 3 discloses a method for manufacturing a high-tensile strength hot-rolled steel sheet having high magnetic flux density. The technique disclosed in Patent Literature 3 is a method for manufacturing a high-tensile strength hot-rolled steel sheet including heating a steel slab containing, in terms of percent by weight, C: 0.02% or more and 0.06% or less, Si: 0.10% or less, Mn: 0.3% or more and 1.2% or less, S: 0.02% or less, Al: 0.10% or less, N: 0.01% or less, and Ti: 0.05% or more and 0.30% or less to a temperature of 1200° C. or more, performing hot rolling with a hot-rolling finishing temperature within the range of the Ar3 transformation point or more and 900° C. or less, and winding it in the temperature range of 500° C. or more and 650° C. or less. The technique disclosed in Patent Literature 3 can achieve a high-tensile strength hot-rolled steel sheet having a tensile strength TS of 50 kg/mm2 (490 MPa) and a magnetic flux density B100 of 1.8 T or more. The technique disclosed in Patent Literature 3 reduces the content of Si to 0.10% or less and ensures desired high strength through precipitation strengthening by Ti carbide. However, the technique disclosed in Patent Literature 3 contains a large amount of Ti, which makes bainitic ferrite having high dislocation density likely to be produced, degrades magnetic properties, and makes it difficult to ensure sufficient magnetic properties as an iron core of a rotary machine.
Patent Literature 4 discloses a hot-rolled steel sheet for an iron core of a rotary machine that contains, in terms of percent by weight, C: 0.10% or less, Si: 0.5% or less, Mn: 0.2% or more and 2% or less, P: 0.06% or less, S: 0.01% or less, Al: 0.1% or less, N: 0.006% or less, and Ti: 0.02% or more and 0.2% or less, further contains at least one of Mo: 0.7% or less (except for the range of 0.2% or less) and W: 0.15% or less, contains carbide smaller than 10 nm containing at least one of Ti, Mo, and W dispersed in a ferrite structure with a volume fraction of 95% or more, and has a strength of about 590 MPa or more. The technique disclosed in Patent Literature 4 can achieve a high-strength hot-rolled steel sheet that has excellent magnetic properties while having excellent formability and has sufficient properties as an iron core of a rotary machine.